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Abstract

The presence of aqueous organic compounds derived from sedimentary organic
matter has the potential to influence a range of chemical processes in hydrothermal vent
environments. For example, hydrothermal alteration experiments indicate that alteration
of organic-rich sediments leads to up to an order of magnitude more metals in solution
than alteration of organic-poor basalt. This result is in contrast to traditional models for
the evolution of vent fluids at sediment-covered mid-ocean ridge axis environments, and
indicates the fundamental importance of including the effects of organic compounds in
models of crustal alteration processes. However, in order to rigorously constrain their
role in crustal alteration processes, quantitative information on the abundances and
distributions of organic compounds in hydrothermal vent fluids is required. This thesis
was undertaken to provide quantitative information on the distributions and stable carbon
isotopic compositions of several low-molecular weight organic compounds (C1-C4
alkanes, C2-C3 alkenes, benzene and toluene) in fluids collected in July, 2000, at three
sites on the northern Juan de Fuca Ridge: the Dead Dog and ODP Mound fields, which
are located at Middle Valley, and the Main Endeavour Field, located on the Endeavour
segment.
At Middle Valley, the ridge axis is covered by up to 1.5 km of hemipelagic
sediment containing up to 0.5 wt.% organic carbon. The Main Endeavour Field (MEF)
is located approximately 70 km south of Middle Valley in a sediment-free ridge-crest
environment, but previously measured high concentrations of NH3 and isotopically light
CH4 relative to other bare-rock sites suggest that the chemical composition of these fluids
is affected by sub-seafloor alteration of sedimentary material (LILLEY et al., 1993).
Differences in the absolute and relative concentrations of NH3 and organic compounds
and the stable carbon isotopic compositions of the C1-C3 organic compounds suggest that
the three fields represent a continuum in terms of the extent of secondary alteration of the
aqueous organic compounds, with the Dead Dog fluids the least altered, the MEF fluids
the most altered and ODP Mound fluids in an intermediate state. At the two Middle
Valley sites, the greater extent of alteration in the ODP Mound fluids as compared to the
Dead Dog fluids is due either to higher temperatures in the subsurface reaction zone, or a
greater residence time of the fluids at high temperatures. Higher reaction zone
temperatures at the ODP Mound field than at the Dead Dog field are consistent with
differences in endmember C1 concentrations between the two fields. The greater extent
of alteration in the MEF fluids is caused by relatively oxidizing conditions in the
subsurface reaction zone that promote faster reaction kinetics.
Temperatures in the subsurface reaction zones calculated by assuming
equilibrium among aqueous alkanes, alkenes and hydrogen are consistent with other
inorganic indicators (C1 and Si concentrations) of temperature, indicating that metastable
equilibrium among these compounds may be attained in natural systems. Isotopic
equilibration among CH4 and CO2 appears to have been attained in ODP Mound fluids
due to the high temperatures in the subsurface reaction zone and the approach to chemical
equilibrium from excess methane. However, isotopic equilibrium between CH4 and CO2
was not attained in the MEF fluids, due to a short residence time of the fluids in the crust
following late-stage addition of magmatic-derived CO2 to the fluids.
Time series analysis indicate that Middle Valley fluid compositions are generally
characterized by stable concentrations over the last decade. However, decreases in Br
concentrations in Dead Dog fluids from 1990 to 2000 suggest that either a greater
proportion of the fluids interact with basalt rather than sediments or that the sediment
with which hydrothermal fluids interact is becoming exhausted. In contrast, the
concentrations of H2 and H2S and the δ34S of H2S are quite different in fluids sampled
from vents of differing ages at the ODP Mound field, despite their close spatial
proximity. The observed variations are caused by the reaction of hydrogen-rich fluids
within the ODP Mound massive sulfide to reduce pyrite to pyrrhotite during upflow. The
replacement of pyrite by pyrrhotite is opposite to the reaction predicted during the
weathering of sulfide minerals weather on the seafloor and reflects the real-time
equilibration of the reduced fluids with mound mineralogy due to the very young age (<2
years) venting from Spire vent. The presence of aqueous organic compounds therefore
affects not only the inorganic chemical speciation in vent fluids, but can also control the
mineralogy of associated sulfide deposits. These results also indicate that vent fluid
compositions do not necessarily reflect conditions in the deep subsurface, but can be
altered by reactions occurring in the shallow subsurface.

Description

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2003

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